[libcxx] Use C++14 when building libc++ with musl

[phosek]

Summary

musl's pthread implementations use volatile types in their structs which is not being constexpr in C++11 but is in C++14.

[phosek]

libc++ was already building with musl after D21637 was commited, but this broke recently as a consequence of r282640 which introduced the use of _LIBCPP_SAFE_STATIC for libc++ internal globals. I'm not sure if this is the best solution, but it's the only one I could come up with, apart from removing the use of _LIBCPP_SAFE_STATIC, which is undesirable, or removing the use of volatile from musl pthread structs, which is unlikely.

[EricWF]

musl's pthread implementations use volatile types in their structs which is not being constexpr in C++11 but is in C++14.

This means that libc++'s std::mutex is unsafe to use during dynamic initialization in C++11 with MUSL, which would really suck except that Clang emits always emits the initialization as a constant expression, ever if it's not guaranteed by the core language.

@mclow.lists Do you have any issue bumping the std version while building for MUSL only?

CMakeLists.txt

327 ↗

(On Diff #74305)

Why not just set LIBCXX_STANDARD_VER differently instead of replacing it after the fact?

[phosek]

CMakeLists.txt

327 ↗

(On Diff #74305)

I totally missed it; this change was a part of a downstream patch we were using for building Fuchsia toolchain and it predates this option. Using this option, I can override the dialect only for our build, which is perfectly fine for Fuchsia since we default to C++14. I'd be happy to abandon this patch unless you want to persist that setting for musl?

[EricWF]

CMakeLists.txt

327 ↗

(On Diff #74305)

Since we support MUSL it would be nice if libc++ built out of the box. Making the option persistent for MUSL makes the most sense to me.

[hfinkel]

CMakeLists.txt

327 ↗

(On Diff #74305)

We should add a comment here, or where ever this logic ends up going, to explain why this is needed.

[phosek]

Ping, do you have any other comments?

[hfinkel]

In D25491#571003, @phosek wrote:

Ping, do you have any other comments?

Fine by me. Please wait for an okay by @EricWF .

[phosek]

@EricWF is this fine with you?

[EricWF]

LGTM.

CMakeLists.txt

331 ↗

(On Diff #74439)

This should probably overwrite the cache entry.

libcxx/trunk/CMakeLists.txt

 
 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
 # Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors
 # so they don't get transformed into -Wno and -errors respectivly.
 remove_flags(-Wno-pedantic -pedantic-errors -pedantic)
 
 # Required flags ==============================================================
 set(LIBCXX_STANDARD_VER c++11 CACHE INTERNAL "internal option to change build dialect")
+if (LIBCXX_HAS_MUSL_LIBC)
+  # musl's pthread implementations uses volatile types in their structs which is
+  # not a constexpr in C++11 but is in C++14, so we use C++14 with musl.
+  set(LIBCXX_STANDARD_VER c++14 CACHE INTERNAL "internal option to change build dialect")
+endif()
 add_compile_flags_if_supported(-std=${LIBCXX_STANDARD_VER})
 mangle_name("LIBCXX_SUPPORTS_STD_EQ_${LIBCXX_STANDARD_VER}_FLAG" SUPPORTS_DIALECT_NAME)
 if (NOT MSVC AND NOT ${SUPPORTS_DIALECT_NAME})
   message(FATAL_ERROR "C++11 or greater is required but the compiler does not support ${LIBCXX_STANDARD_VER}")
 endif()
 
 # On all systems the system c++ standard library headers need to be excluded.
 # MSVC only has -X, which disables all default includes; including the crt.